1. Field of the Invention
The present invention relates to a case nitrided product, a process for producing a case nitrided product, and a nitriding agent used for nitriding an aluminum product.
2. Description of the Related Art
Aluminum products have been well known that they exhibit a hardness smaller than that of steel or the like. When they are slid on the steel or the like, they are extremely likely to be seized and worn out. Therefore, they have been investigated for their applicability to a variety of surface treatments which utilize, for example, plating, thermal spraying, anode oxidizing and the like. Some of the treatments have been put into actual applications. Most of these treatments form an oxide layer on the surface of the aluminum products. There were a few try-outs by nitriding, however, the resulting nitriding layers formed on the surface of the aluminum products were so thin that no satisfactory case nitrided aluminum products have been available so far. Since these trial nitriding processes have required expensive equipment capable of producing a high degree of vacuum or the like, none of the processes have been put into actual application.
As recited in Japanese Unexamined Patent Publication (KOKAI) No. 60-211,061, there has been reported recently a process for forming a nitriding layer on the surface of the aluminum products. According to the process, prior to nitriding step, a pro-sputtering step is carried out in an argon gas atmosphere which contains a nitrogen or oxygen gas in a trace amount, and then a nitriding step is carried out by ion nitriding in which glow discharge is effected in a nitrogen gas atmosphere. Further, as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 63-290,255, there is disclosed a surface treatment process for an aluminum product which utilizes a nitrogen ion injection. Furthermore, in Japanese Unexamined Patent Publication (KOKAI) No. 62-153,107 and Japanese Unexamined Patent Publication (KOKAI) No. 62-278,202, as a direct nitriding process for aluminum, there are disclosed examples of directly nitriding granular aluminum, however, there are no descriptions that the nitriding can be used as a surface treatment.
Aluminum has a melting point of 650.degree. C. The melting point is lower by a factor of about 1/3 than that of steel (e.g., about 1,600.degree. C). Accordingly, during the surface treatments which are carried out below the melting point either by means of aluminum oxide layer formation or aluminum nitride formation, it has been regarded inevitable that the film forming rate is extremely slow. Further, aluminum is very active and is likely to be oxidized. Consequently, on the surface of aluminum, there always exists a natural oxide layer slightly occupying a part of the area thereof. The oxide layer inhibits the nitriding layer from forming. Furthermore, even if the oxide layer can be removed by sputtering or the like prior to surface treatment, aluminum is oxidized preferentially in commercially available apparatuses which produce a vacuum on the order of 10.sup.-5 Tort vacuum degree. Hence, it has been said that aluminum is hardly nitrided.